1. Field of the Invention
The present invention relates generally to electronic measuring systems, and more particularly to load indicator conversion systems for converting a scale having a mechanical measuring system into a scale having an electronic measuring system in which the load applied to the mechanical load receiver is converted into an electronic display.
2. Description of the Prior Art
Mechanical scales in which a load is applied to a mechanical load receiving member, such as a calibrated spring assembly and platform, in order to mechanically control a balance mechanism in order to mechanically provide a weight indication, such as in a conventional portable beam scale, bench scale, or health scale are well known in the art. In addition, electronic systems which measure weight to provide an electronic display based on a load applied to a transducer, such as an analog load cell or a digital load cell, such as disclosed in U.S. Pat. Nos. 3,850,023; 3,665,169; 3,603,298; 4,330,837; 4,143,724; 4,722,406; and 4,137,568 are also well known in the art, as are the use of electronic checkweighers, such as disclosed in U.S. Pat. Nos. 4,321,439; 3,446,299; 3,770,969; and 4,363,408. However, applicants are not aware of any systems which can efficiently, and relatively inexpensively, convert conventional existing mechanical scale into a modern electronic scale so as to provide an electronic display of the magnitude of the load applied to the existing mechanical load receiving member and/or to provide an electronic checkweigher from the existing mechanical scale. This is particularly important in commercial environments where costly mechanical scales are still functional and need not be discarded in order to achieve all of the benefits and advantages desired and needed today in a modern electronic measuring system.
Typically, in prior art electronic scales, a continuous voltage is applied across a load cell arranged in a wheatstone bridge. The voltage across the load cell output is measured, from which the magnitude of the load applied to the load cell is determined. Many of these prior art electronic scales have been battery operated. In early electronic scales of this type, the electrical components and the load cell each used comparatively large amounts of power. With the advent of digital circuitry, the power consumption of the logic components begin to shrink as compared to the load cell and systems were developed to reduce load cell power consumption, such as disclosed in U.S. Pat. No. 4,238,784 which employs an electronic measuring system with a constant dual-frequency pulsed power supply for pulsing the power at the load cell input, thereby utilizing less power than with a continuous D.C. power supply.
However, as noted in the above referred to contemporaneously filed copending patent application, even with such prior art pulsed systems, the zero reading of the transducer tends to drift. This effect is ordinarily caused by temperature changes in the system and by residual voltages in the system during a no pulse cycle. Various techniques have been applied to compensate for drift, such as disclosed in U.S. Pat. No. 4,238,784, however, although satisfactory in some circumstances, such prior art techniques do not enable the pulses to be varied with respect to duty cycle and frequency to thereby further reduce power consumption to lower desired levels.
In order to try to compensate for effects caused by temperature changes in prior art battery operated scales, separate temperature sensors have been employed for separately housed detection and display circuitry, as discussed in the aforementioned copending contemporaneously filed patent application. However, such a prior art arrangement has not only resulted in increased cost, but has also resulted in reduction in correction accuracy.
In the past, when prior art electronic scales were used as checkweighers, they generally incorporated separate display elements to indicate an over/under condition, such as a series of LED's. However, such a prior art arrangement not only adds to the cost of the scale by requiring additional display elements and space on the display face, but the LED's employed also increase the power consumed by the system and further exacerbate the unwanted problem of heat in the casing.
These prior art difficulties with electronic scales, and particularly battery operated electronic scales, having further added to the difficulty in providing an efficient low cost alternative to the previously acceptable mechanical scales which have not as yet outlived their useful life. These disadvantages of the prior art are overcome by the present invention.